Fill a large pot with salted water and bring it to a boil. Add the penne pasta and cook according to package instructions until al dente, about 9-11 minutes. Drain the pasta and set aside, reserving a little pasta water for the sauce if needed.
While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced onion and cook until softened and translucent, about 3-4 minutes. Stir in the minced garlic and cook for another 30 seconds until fragrant.
Add the sliced chicken sausage to the skillet. Cook for about 5-6 minutes, stirring occasionally, until the sausage is browned and cooked through.
Pour in the entire can of diced tomatoes with their juices. Stir in the Italian seasoning, salt, and pepper to taste. Let the mixture simmer for 5 minutes to allow the flavors to meld and the sauce to slightly reduce.
Add the chopped fresh spinach to the skillet and stir until it wilts down, about 1-2 minutes. Then pour in the heavy cream, stirring to combine everything into a creamy sauce. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it up.
Add the cooked penne pasta directly into the skillet, tossing everything together until the pasta is evenly coated with the sauce. Taste and adjust seasoning with extra salt and pepper if needed.
Spoon the pasta onto plates or into bowls. Sprinkle generously with grated Parmesan cheese for extra richness and serve immediately while warm.
